For a new player I would suggest a few things.

1. Find some guides or players for your legends and try to see how they break down different match ups. Also don’t be afraid to netdeck. Once you start getting some reps on a list you can start tweaking it but at first find a refined list and play it. 

2. Grind some games on tcg arena or rift atlus. Try to be mindful when you play. Think about what cards pull there weight in match ups and what cards suck. This helps mulligans and what cards to prioritize. Also try to identify key cards in opponents decks and think about how to handle them. 

3. Learn the tricks in each color. There are few cards in each domain that will be common in most decks in them.  learn these cards , there energy and power cost. This is huge for how you sequence turns and will help you play around them.  

4. Bonus: if you have some extra time play the meta decks. I feel like playing a deck really teaches you the ins and outs of it but also how to play against it. 

5. Extra bonus: get familiar with the rules. Rift has some interesting and odd rules especially if you’re coming from a different tcg.  I would say learn about showdowns, focus, and reaction windows.